Users have long desired lighter weight gun systems that remain durable and reliably accurate. It is known to substitute relatively strong but lightweight materials—such as unreinforced and reinforced polymers, continuous glass fiber or carbon fiber composites—for various portions of the gun commonly fabricated from steel, aluminum, or other metals. Attention has focused on gun barrels, which constitute a large percentage of a gun's weight. It is known, for example, to fabricate a gun barrel having an inner liner, typically a steel alloy, surrounded by a continuous carbon fiber reinforced polymer matrix composite outer shell. With the appropriate choice of materials and properly engineered, this combination lightens the gun while retaining good barrel strength and stiffness.
The carbon fibers used in the outer shell may be any types that provide the desired stiffness, strength and thermal conductivity. Typically for gun barrel applications, polyacrylonitrile (“PAN”) precursor or pitch precursor carbon fibers are used. The carbon fiber may be applied in a wet filament winding operation, wherein dry carbon fiber strands or tows are combined with a resin in a “wet” dip pan process, then wound around the inner liner and processed. Alternatively, the shell may be fabricated from carbon fiber tow, unidirectional tape, or fabric that was previously impregnated with resin in a separate process (“towpreg” or “prepreg”), or a textile preform wherein the resin is infused into the braided preform, then applied to the inner liner in a process that cures the prepreg into a hard thermally stable matrix and simultaneously bonds the outer shell to the barrel inner liner. Whether applied by wet filament winding, resin infusion into a dry preform, or by application of prepreg materials, the matrix resin is typically a crosslinkable epoxy, but the resin may be a polymer such as a polyimide, bismaleimide, cyanate ester, inorganic polymer, thermoplastic polymer, or some other material as the inventors described in patent application PCT/US14/53194 (Curliss), the specification and drawings of which are hereby incorporated in their entirety. The matrix binder may not be an organic polymer resin at all, but may be an inorganic polymer, a metal, a ceramic, allotropes of carbon, or a mineral. The composite barrel may then be cured (where relevant), finished, and attached to a receiver and stock. Such carbon fiber reinforced composites can provide a suitable balance of thermal properties, mechanical properties, and processing characteristics for many common firearms applications. Other fibers known to those skilled in the art, including continuous glass fibers, continuous ceramic fibers, continuous metallic fibers, continuous graphite fibers, continuous mineral fibers, continuous polymer fibers and/or combinations thereof may also be used as the reinforcement phase.
Such composite gun barrels, however, can pose problems not encountered with traditional steel barrels. First, the composite must be constructed in a manner and quantity around and along the liner to ensure that the barrel does not burst upon firing, to achieve satisfactory strength and stiffness in the principal directions (e.g., axially and torsionally), to provide adequate environmental durability, and to dampen the shock wave that propagates when the projectile is fired. For example, dampening of the shock wave through reflection, refraction, and interaction in inhomogeneous materials will vary depending on material properties, such as fiber diameter and geometric orientation, and volume fraction of the continuous fibers within the matrix.
Most of the foregoing issues can be addressed by additional windings, e.g., more circumferential “hoop wraps” to improve burst strength and more axially oriented helical windings to improve axial tensile and flexural strength and stiffness. Torsional stiffness is a significant design factor important in medium and large caliber barrels having rifling. However, adding more layers of windings can lead to manufacturing and curing complications, higher material expense, more weight, and a bulkier barrel profile than desired. Fiber selection can also address these problems to some extent. Generally lower density, stronger and stiffer fibers are preferred provided they do not exhibit other undesirable characteristics, such as poor resin adhesion.
Second, thermal management is a significant concern, inasmuch as the more common continuous fiber composite (“CFC”) outer shells are relatively poor conductors of the heat generated by hot gasses within the liner. Additional layers of CFC windings exacerbate the heat removal problem. During operation, the barrel will heat up. In the case where the matrix phase is an organic polymer, if the cured resin within the CFC reaches its glass transition temperature, Tg, the CFC softens significantly and the mechanical integrity of the composite barrel is compromised. As the barrel is heated to even higher temperatures, irreversible thermal decomposition of the cured matrix occurs and barrel structural integrity is further compromised. U.S. Pat. No. 6,889,464 (Degerness) added a thermally conductive material to the resin mixture to improve thermal conductivity and heat dissipation. Curliss, supra, (PCT/US14/53194) disclosed a novel method for manufacturing gun barrels using resins that withstand higher temperatures, and disclosed using small particles of metal such as aluminum as a thermal conducting additive.
A third problem relates to stresses within the barrel arising from thermal expansion differences between the composite and the inner liner of the composite barrel. As the inner steel liner heats during operation, it expands both radially and longitudinally. Composite structures in the prior art have a substantially lower average effective coefficient of thermal expansion (CTE) in the longitudinal direction than steel and so when heated, the CFC outer shell expands substantially less than the steel liner. This may increase or decrease thermal stresses in the barrel depending on the state of thermal residual stress from processing. The point is that as the temperature changes in the barrel, due to operation or the environment, the state of residual stress in the barrel also changes. For example, the CTE of type 416 grade stainless steel, an alloy commonly employed in steel gun barrels, is about 5.55 parts per million per degree Fahrenheit (5.55 ppm/° F., or 5.55×10−6/° F.), while the longitudinal average effective CTE for a typical CFC outer shell employing PAN precursor carbon fiber and a thermoset epoxy resin is less than about 3 ppm/° F. When a type 416 stainless steel liner and a typical CFC are subjected to heating during operation, uneven expansion can produce thermal stresses on the liner-CFC interface, possibly even causing separation of the CFC from portions of the liner or fractures within the CFC shell. Even if no separation occurs, minor variations in the CFC or metal liner properties, or geometric variation, may promote uneven thermal stresses at the interface between the barrel and CFC that may result in nonlinear deformation or displacement of the barrel from its original axis. Even a very slight displacement can significantly degrade accuracy. Moreover, even if the barrel and liner remain perfectly true, the various layers of windings within the CFC can have different CTEs, especially longitudinally. When subjected to elevated operating temperatures, differences in the thermal expansion of adjacent winding layers within the CFC can result in high levels of interlaminar shear stress and even delamination.
U.S. Pat. No. 5,692,334 (Christensen) disclosed eliminating any bond or adhesion between the inner liner and the CFC. Unfortunately, this approach virtually eliminates any contribution of the outer shell to axial stiffness, torsional stiffness, or circumferential reinforcement. The same inventor in U.S. Pat. No. 5,804,756 recognized that steel and the composite shell have different CTEs, but attempted to match thermal expansion only in the radial direction. Indeed, one object of the '756 patent is to “have nearly 0 coefficient of thermal expansion in the axial direction.” The '756 patent expressly teaches that reducing the CFC's expansion to zero in the axial direction improves accuracy. '756 patent col. 2, line 23; col. 6 line 11.
U.S. Pat. No. 5,600,912 (Smith) teaches mechanical compression of the carbon fiber composite outer shell longitudinally after it is cured to improve barrel stiffness, which compression could also help compensate for a lower CFC thermal expansion when the barrel is heated during operation. However, mechanically compressing the CFC risks damage e.g., through over tightening, and in any case the “proper” amount of cold residual compression to apply will vary depending on the barrel's operating temperature as well as structural characteristics such as barrel length and liner profile. Like Smith, U.S. Pat. No. 6,189,431 (Danner) also mechanically exerts residual cold compression on the CFC, but it is accomplished by means of steel flanges on the liner ends which compress the CFC as the steel liner contracts more than the CFC during the cooling phase of the curing process. Like Smith, Danner does not address the underlying problem of mismatched CTEs, and seems to accept as a given that a steel liner inherently has a higher CTE than a continuous fiber composite. Moreover, Danner continues the prior art of abruptly alternating winding angles between layers.
Producing an optimized composite barrel must balance competing considerations. What is needed is a carbon fiber composite projectile barrel that employs reasonably priced materials, that provides superior axial and torsional strength and stiffness while minimizing weight and radial bulk, that minimizes interlaminar stress, and that does not deform when heated due to mismatched axial CTEs between the liner and outer shell.